1. Field of the Invention
This invention relates generally to simplified feedthrough filter capacitor assemblies and related methods of construction. Feedthrough filter capacitor assemblies are used to decouple and shield undesirable electromagnetic interference (EMI) signals from implantable medical devices and other electronic devices.
More specifically, this invention relates to simplified and reduced cost filter capacitors for ceramic feedthrough terminal pin assemblies. In the present invention, the filter capacitor assembly is mounted to the ceramic feedthrough terminal pin assembly. The ceramic feedthrough is used to connect a terminal pin or electrode through a hermetically sealed housing to internal electronic components of the medical device while the filter capacitor decouples EMI signals against entry into the sealed housing via the terminal pin. This invention is particularly designed for use in cardiac pacemakers (bradycardia devices), cardioverter defibrillators (tachycardia), neurostimulators, internal drug pumps, cochlear implants and other medical implant applications. This invention is also applicable to a wide range of other EMI filter applications, such as military or space electronic modules, where it is desirable to preclude entry of EMI signals into a hermetically sealed housing containing sensitive electronic circuitry.
In that respect, feedthrough terminal pin assemblies are generally well known in the art for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices such as cardiac pacemakers, defibrillators, and the like, the feedthrough assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known in the art for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of body fluids into the housing thereof. However, the feedthrough terminal pins are typically connected to one or more lead wires leading to a body organ such as a heart. The lead wires effectively act as an antenna and tend to collect stray EMI signals for transmission into the interior of the medical device. That is why hermetic feedthrough assemblies are combined with a ceramic feedthrough filter capacitor to decouple interference signals to the medical device housing. However, a primary feature of the simplified feedthrough filter capacitor described herein is volume reduction without compromising effectiveness and reliability. This is accomplished by elimination of some of the volume previously occupied by the capacitor dielectric using new screen printing techniques. The present feedthrough filter capacitor is also less costly to manufacture than a comparably rated prior art capacitor.
2. Prior Art
In a typical prior art unipolar construction for a feedthrough filter capacitor, such as described in U.S. Pat. No. 5,333,095 to Stevenson et al., a round/discoidal (or rectangular) ceramic feedthrough filter capacitor is combined with a hermetic feedthrough terminal pin assembly. In use, the coaxial capacitor permits passage of relatively low frequency electrical signals along the terminal pin while shielding and decoupling/attenuating undesired interference signals of relatively high frequency to the conductive housing of the medical device.
One type of hermetic feedthrough terminal pin subassembly widely used in implantable medical devices employs an alumina ceramic insulator which, after sputtering/metallization procedures, is gold brazed into a titanium ferrule. In addition, there are terminal pins, typically made of platinum, which are also gold brazed to the alumina ceramic insulator to complete the hermetic seal. See for example, the subassemblies disclosed in U.S. Pat. Nos. 3,920,888; 4,152,540; 4,421,947 and 4,424,551. Separately, the feedthrough filter capacitor is constructed by preassembly of the coaxial capacitor and then mounting it onto or within the cylindrical or rectangular hermetically sealed feedthrough terminal pin subassembly including the conductive pins and ferrule.
The feedthrough capacitor is of a coaxial construction having two sets of electrode plates embedded in spaced relation within an insulative dielectric substrate or base. The dielectric base is typically formed as a ceramic monolithic structure. One set of “active” electrode plates is electrically connected at an inner diameter cylindrical surface to a terminal pin in a unipolar (one terminal pin) construction. Feedthrough capacitors are also available in bipolar (two), tripolar (three), quadpolar (four), pentapolar (five), hexpolar (six), and additional terminal pin configurations. The inner active plates are coupled in parallel together by a metallized layer which is either glass frit fired or plated onto the ceramic capacitor. This metallized band, in turn, is mechanically and electrically coupled to the terminal pin by a conductive adhesive or soldering, and the like.
The other or second set of “ground” electrode plates is coupled at an outer diameter surface of the discoidal capacitor to a cylindrical ferrule of conductive material. The outer ground plates are coupled in parallel together by a metallized layer which is fired, sputtered or plated onto the ceramic capacitor. This metallized band, in turn, is coupled to the ferrule by conductive adhesive, soldering, brazing, welding, and the like. The ferrule is electrically connected in turn to the conductive housing of the electronic device.
The device housing is constructed from a biocompatible metal such as of a titanium alloy, which is electrically and mechanically coupled to the hermetic feedthrough terminal pin subassembly, which is, in turn, electrically coupled to the feedthrough filter capacitor. As a result, the filter capacitor coupled to the feedthrough terminal pin subassembly prevents entrance of interference signals to the interior of the device housing, where such interference signals could otherwise adversely affect the desired device function such as cardiac pacing or defibrillation.
Although feedthrough filter capacitor assemblies of the type described perform in a generally satisfactory manner, the associated manufacturing and assembly costs are unacceptably high. One area where costs can be reduced is in the manufacture of the feedthrough filter capacitor.
FIG. 1 illustrates a typical prior art filter feedthrough capacitor assembly 10 comprising a filter capacitor 12 mounted to a feedthrough terminal pin subassembly 14. The filter feedthrough capacitor assembly 10 is shown in one preferred form comprising a so-called bipolar configuration having two separate conductive terminal pins 16 extending through the discoidal-shaped filter capacitor 12 and feedthrough terminal pin subassembly 14.
The feedthrough terminal pin subassembly 14 comprises a ferrule 18 defining an insulator-receiving bore 20 surrounding an insulator 22. Suitable electrically conductive materials for the ferrule substrate 18 include titanium, tantalum, niobium, stainless steel or combinations of alloys thereof. Titanium is preferred for the ferrule 18, which may be of any geometry, non-limiting examples being round, rectangle, and oblong. A surrounding inwardly facing annular channel 24 is provided in the ferrule 18 to facilitate attachment of the filter feedthrough capacitor assembly 10 to the casing 26 of, for example, the implantable medical device. The method of attachment may be by laser welding or other suitable methods.
The insulator 22 is of a ceramic material such as of alumina, zirconia, zirconia toughened alumina, aluminum nitride, boron nitride, silicon carbide, glass or combinations thereof. Preferably, the insulating material 22 is alumina, which is highly purified aluminum oxide, and comprises a sidewall 22A extending to a first upper surface 22B and a second lower surface 22C. A layer of metal 28, referred to as metallization, is applied to the sidewall 22A of the insulating material 22 to aid in the creation of a brazed hermetic seal. Suitable metallization materials 28 include titanium, niobium, tantalum, gold, palladium, molybdenum, silver, platinum, copper, carbon, carbon nitride, titanium nitrides, titanium carbide, iridium, iridium oxide, tantalum, tantalum oxide, ruthenium, ruthenium oxide, zirconium, and mixtures thereof. The metallization layer 28 may be applied by various means including, but not limited to, sputtering, e-beam deposition, pulsed laser deposition, plating, electroless plating, chemical vapor deposition, vacuum evaporation, thick film application methods, aerosol spray deposition, and thin cladding.
The insulator 22 has a sufficient number of bores 30 to receive the terminal pins 16. The inner surfaces of these bores 30 are provided with a metallization layer 32 in a similar manner as the previously described insulator sidewall 22A. The terminal pins 16 are then received in the bores 30. Preforms (not shown) of a conductive, biostable material, such as gold or gold alloy, are moved over the terminal pins 16 to rest against the upper insulator surface 22B adjacent to annular notches 34 in the insulator. Similarly, a gold preform (not shown) is positioned at the junction of the ferrule insulator-receiving bore 20 and the upper insulator surface 22B. The thusly-assembled feedthrough terminal pin assembly 14 is then heated in an oven or furnace to melt the preforms and cause them to form their respective brazes 36 and 38. Braze 36 hermetically seals the terminal pins 16 to the insulator 22 at the terminal pin bores 30 while braze 38 hermetically seals the insulator 22 to the ferrule 18 at the insulator-receiving bore 20.
The feedthrough filter capacitor 12 comprises a dielectric 40 formed from multiple layers of a tape cast ceramic or ceramic-based material containing multiple capacitor-forming conductive first “active” electrode layers 42 and second “ground” electrode layers 44 screen-printed in an alternating manner on top of the tape cast dielectric. This layered assembly is then sintered to provide a monolithic dielectric body containing the electrode layers 42, 44. Although the exemplary drawing shows in exaggerated scale a pair of the active electrode layers 42 in parallel staggered relation with a corresponding pair of the ground electrode layers 44, it will be understood that a large plurality of typically 5 to 40 conductive layers 42 can be provided in alternating stacked and parallel spaced relation with a corresponding number of the conductive layers 44.
Each of the active electrode layers 42 is subdivided into two spaced-apart and generally pie-shaped electrode plates (not shown). Accordingly, the two electrode plates 42 of each layer group are electrically insulated from each other by the dielectric material 40 of the capacitor 12. The multiple spaced-apart layers of the active electrode plates 42 are formed in stacked alignment with the respective active electrode plates 42 of overlying and underlying layers to define two respective active plate stacks. The two terminal pins 16 pass generally centrally through respective bores 46 formed in these active plate stacks, and are conductively coupled to the associated stacked set of active electrode plates 42 by a suitable conductive surface lining such as a surface metallization layer 48 lining each bore 46.
A plurality of spaced-apart layers of the second or “ground” electrode plates 44 are also formed within the capacitor 12. The ground electrode plates 44 are in stacked relation alternating or interleaved with the layers of active electrode plates 42. These ground electrode plates 44 include outer perimeter edges which are exposed at the outer periphery of the dielectric body 18 where they are electrically connected in parallel by a suitable conductive surface such as a surface metallization layer 50. Importantly, however, the outer edges of the active electrode plates 42 terminate in spaced relation with the outer periphery of the capacitor body 12. In that manner, the active electrode plates 42 are electrically isolated by the dielectric body 40 from the conductive layer 50 coupled to the ground electrode plates 44. Similarly, the ground electrode plates 44 have inner edges which terminate in spaced relation with the terminal pin bores 46, whereby the ground electrode plates 44 are electrically isolated by the dielectric body 40 from the terminal pins 16 and the conductive metallization layer 48 lining the pin bores 46. The number of active and ground electrode plates 42 and 44, together with the dielectric 18 thicknesses or spacing there between may vary in accordance with the desired capacitance value and voltage rating.
The feedthrough capacitor assembly 10 is constructed by moving the filter capacitor 12 over the feedthrough terminal pin subassembly 14. A non-conductive disk-shaped member 52 is positioned about the terminal pins 16 at a location sandwiched between the upper insulator surface 22B of the feedthrough terminal pin subassembly 14 and the bottom of the capacitor terminal pin bores 46. In this position, the non-conductive disc 52 supports the capacitor 12 in spaced relation above the insulating material 22. The metallized surface 48 within the terminal pin bores 46 is then connected electrically to the terminal pins 16 by means of a conductive adhesive bead 54, or by soldering or brazing or the like. In a preferred form, the conductive adhesive 54 is applied to the annular gap between the pins 16 and the capacitor metallized surface 48, and allowed to fill a portion (about one-half) of the gap length. Similarly, the metallized surface 50 associated with the ground electrode plates 44 of the capacitor 12 is connected electrically to the ferrule 18 by means of an additional fillet 56 of conductive adhesive or the like. One preferred conductive adhesive comprises a curable polyimide adhesive loaded with conductive particles such as spheres or flakes, as described by way of example in U.S. Pat. No. 4,424,551, which is incorporated by reference herein. However, it will be understood that other conductive connecting means may be used, such as solder, braze or the like. Importantly, the adhesive beads 54, 56 establish an electrically conductive mounting of the capacitor 12 in a secured stable manner to the feedthrough terminal pin assembly 14.
Mechanically, the barium titanate material typically used as the capacitor dielectric material 40 is relatively weak and prone to fracture. Also, if the dielectric material 40 is not sufficiently thick, it tends to warp into a “potato chip” shape upon being heated during sintering of the tape cast material. That is why in the prior art filter feedthrough capacitor assembly 10, the thickness of the respective lower and upper dielectric zones 58 and 60 below and above the intermediate zone occupied by the electrode layers 42, 44 is from about 0.007 inches to 0.015 inches, preferably about 0.010 inches. In other words, the thickness from the lower surface of the dielectric 18 adjacent to the non-conductive disc 52 to the lowest ground plate 44 is about 0.010 inches and the distance from the upper active plate 42 to the upper surface of the dielectric body 18 also about 0.010 inches. Depending on the number of active and ground electrode plates and the spacing between them, the intermediate zone in the dielectric body supporting the electrode plates 42, 44 is typically a minimum of about 0.010 inches, to much more. Therefore, the capacitor 12 is generally of a minimum thickness of about 0.030 inches and in certain applications can be significantly greater. The bulk of this thickness is occupied by the lower and upper dielectric zones 58, 60 sandwiching the intermediate zone of the electrode plates and each being about 0.010 inches thick. If these dielectric zones 58, 60 could be made thinner without compromising function and reliability including planarity, significant size reductions could be realized.
Accordingly, there is a need for a novel feedthrough filter capacitor assembly that addresses the drawbacks noted above in connection with the prior art. In particular, a novel capacitor assembly is needed which significantly reduces the volume occupied by this assembly, or is of a comparable volume, but of a significantly higher capacitance rating, without any diminution in filtering performance or reliability and yet that may be utilized in many of the same applications where such subassemblies are now found. Additionally, the improved feedthrough filter capacitor assembly should lend itself to standard manufacturing processes such that cost reductions can be realized immediately. Of course, the new design must be capable of effectively filtering out undesirable electromagnetic interference (EMI) signals from the target device. The present invention fulfills these needs and provides other related advantages.